The present invention relates to a semiconductor device, a portable communication terminal, an IC card, and a microcomputer having a near field communication (NFC) function using electromagnetic induction.
In a noncontact communication system, a noncontact communication card operates synchronously with a clock signal extracted from a carrier wave supplied from a reader/writer. Therefore, when the carrier wave disappears during communication, the clock signal cannot be extracted, and the operation of the communication card is stopped. For example, in the case of inputting an ASK (Amplitude Shift Keying) 100% modulation signal whose modulation target is 100% of amplitude from an antenna, the carrier wave disappears during an input signal modulation period, and the clock signal cannot be extracted. It is also assumed that the carrier wave disappears for other reasons.
Japanese Unexamined Patent Publication No. 2007-102383 discloses a technique that, when a carrier wave breaks in a communication of type A using an ASK 100% modulation signal, an internal clock is temporarily stopped, thereby adjusting the operations of internal circuits such as a decoding circuit both at the time of reception of an ASK 100% modulation signal and at the time of reception of an ASK 10% modulation signal.